1. Field of the Invention
The present invention relates to a vehicular air conditioning system for controlling the number of revolutions per unit time of a compressor to make the air conditioning capacity variable by controlling an AC motor using an inverter and, more particularly, to a vehicular air conditioning system including an electric heater as a warming heat source for activating the AC motor and the electric heater with the output coming from the inverter by switching an electromagnetic relay.
2. Description of Related Art
Unexamined Published Japanese Patent Application Laid-Open No. 7-304325 (the "published system") discloses a system employing a three-phase AC motor as a motor for driving the compressor which controls the number of revolutions per unit time variably with the inverter.
In the published system, the power to the electric heater is intercepted by means of an inexpensive electromagnetic relay. Here, the electric contacts of the relay are provided at the individual three output lines between the inverter and the three-phase AC motor, either of which is selectively activated by the output coming from the inverter.
In other words, the three-phase AC motor is controlled at a cooling time with the output coming from the inverter, and the electric heater is energized at a warming time by the inverter.
Here, the electric heater can be made small by raising the voltage to a high level (e.g., 300 VDC, for example), and is highly advantageous for a vehicular air conditioning system which is restricted in its mounting space.
Therefore, the inventors of the present invention have examined the actions of the relay for the output voltage of the inverter in the published system. It has been revealed by the examinations that when the relay is activated for a high output voltage coming from the inverter, there arises a problem that the contacts are damaged, thereby deteriorating the durability of the arm with the contacts of the relay being opened.
In order to solve this problem, moreover, it is sufficient to raise the breakdown voltage (or capacity) of the relay. No appropriate relay capable of raising the breakdown capacity sufficiently is commercially available at present. When such an improved relay is to be newly manufactured, there arises another problem that the manufacturing facilities raise the cost.